The solenoid is often combined with a two-position valve, whereby the valve is pulled by the solenoid (when energized) and returned by a spring (when de-energized). The valve attached to the solenoid can be closed in one position and open in the second position, or it can be a changeover valve with two seats. In some applications, such as fuel injectors, it is desirable to measure and control the timing of the opening and closing positions of the solenoid operated valves.
Diesel fuel injectors need to have precise operating times. The valve determines the injection timing and also the injection duration (quantity of fuel) injected into a cylinder of a diesel engine. The performance of the engine (balance between cylinders, power, fuel consumption, emissions) is thus affected.
Known means exist to measure the impact point of the solenoid valve at the end of the energized stroke. For example Woodward U.S. Pat. No. 6,889,121 describes one such means. In this way the first (energized) travel time of the solenoid valve motion can be measured. It is desirable also to measure the impact of the solenoid valve at the end of the de-energized stroke. Thus information relating to the duration of the solenoid operation can be obtained for helping to control the amount of fuel supplied. This is particularly useful when the duration is short (as in diesel pilot injections)—small variations in duration can give large percentage variations in the output of a pilot injection.
A method of measuring the de-energized impact point is described in GB 2 110 373. In this patent specification, the method tries to measure the end of the solenoid movement by detecting a small change in the current to the solenoid caused by the back EMF when the solenoid stops moving (at the end of the de-energized stroke). This is very hard because of the small size of the change. Part of the reason for the small size of the change is that the gap between the solenoid armature and the stator is relatively large after de-energization. Often detecting this small current change is not possible because there is no current flowing at this time, and thus no change can be measured.
A further method is described in U.S. Pat. No. 5,650,909 in which a small current is added (to the solenoid) after the main current has been switched off. This is done to overcome the limitation above. The current must be small so that it does not affect the valve motion. Again there is difficulty in reliably measuring the small changes.
A yet further method involves adding a transducer to measure the valve motion directly. This can be a movement transducer or a pressure transducer (measuring some function of the valve attached to the solenoid). However, adding additional transducers considerably increases the system cost, and also increases complexity.
The Woodward U.S. Pat. No. 6,889,121 describes how to measure the end of the forward (energized) motion of the solenoid, and using this to control solenoid timing. This does not attempt to measure the solenoid return stoke. Changes in the operating conditions or parameters of the valves within a fuel injection system, for example wear, can lead to changes in the time of the valve return stroke. This in turn can lead to changes in the amount of fuel injected for a supply pulse of given duration and thus also to variations in the performance of the valves of a fuel injection system.